1. Field of the Invention
The invention relates to the field of thin film magnetoresistive transducers used for sensing the magnetic field recorded on magnetic recording media.
2. Prior Art
A magnetoresistive element changes resistance as a function of the strength and direction of the magnetic field to which it is exposed. In use, a sense current is applied to the magnetoresistive element. Then, as the magentic medium passes the magnetoresistive element, the resistance of the magnetoresistive element changes in response to the strength and direction of the magnetic field in the medium. The change in resistance produces corresponding voltage changes across the element.
Because of poor sensitivity and nonlinearity of the magnetoresistive effect in the region around zero applied magnetic field, most applications using magnetoresistive elements in a magnetic head require a fixed bias field to be applied by means of either a permanent magnet or an electromagnet in order to increase the sensitivity and linearity of the head.
Permanent magnets can be used to provide the necessary bias field, but for complex configurations, such as multitrack heads, practical designs are difficult to achieve. It is therefore preferable in such applications to use a current induced electromagnetic field for the bias.
One approach to supplying the necessary bias field has been to use a so-called shunt bias. In this arrangement, a magnetoresistive film is placed in electrical contact with a higher resistivity, non-magnetic conductive layer. The magnetoresistive layer is then mangetically biased by the portion of the sense current which is shunted through the nonmagnetic layer. This is the approach used in U.S. Pat. No. 3,813,692 issued to Brock, et al. An inherent characteristic of the shunt bias approach is that the sense and bias currents are not independently controllable, the relative amount of bias current being determined by the resistivity of the selected shunt material. In addition, the use of the shunt bias results in a loss of as much as 60% of the possible output signal. Also, excessive thermal noise can result from the use of some materials as the shunt, e.g., titanium.
Other techniques include the so-called barber pole configuration which avoids the need for a bias by driving the sense current at an angle to the sense element itself. The drawback of this configuration is that it is relatively complex lithographically and is not easily adaptable to small shielded sensor elements. Another technique, known as self-bias, uses a magnetoresistive element positioned non-symetrically in the gap of a shielded head. Because of preferential flux coupling to the closest shield, the needed bias is self-induced by the sense current flowing through the element.